One of the most active fields within analytical chemistry is ambient desorption/ionization-mass spectrometry (ADI-MS) due to the capacity to “analyze things as they really are”. A plethora of ADI source designs have been described in the literature, with most seeking to affect the sampling (volatilization) of a solid under ambient conditions with subsequent ionization in the gas phase by the same or a supplemental form of energy. The most evolved of these approaches include desorption electrospray ionization (DESI) and direct analysis in real time (DART) sources. These approaches typically include low temperature (<100° C.) volatilization of molecular species from a surface such that there is minimal thermal degradation and soft ionization via proton addition, with each component affected by different processes. The sampling is fairly diffuse, not having a high level of lateral resolution/selectivity in comparison to microprobe methods. Regiospecific sampling can be affected through the use of other steps, such as a laser vaporization step.
The use of the liquid sampling-atmospheric pressure glow discharge device (LS-APGD) as an ionization source for analysis via mass spectrometry has been demonstrated in both an ADI-MS mode and as a molecular ionization source. The LS-APGD has been recognized as a low power, low sample consumption microplasma source alternative to the inductively coupled plasma (ICP), and has been a successful source for miniaturized instrumentation platforms.
Independent of the energy source(s) for the sampling and ionization components of the ADI-MS process, once the sample is volatized and ionized, it is characterized by mass spectrometry, which requires the ionic species of the sample to be extracted and directed to the analyzer. Unfortunately, use of mass spectrometry requires that the analytical process be a multi-step process in which the sample is initially volatized and ionized (which can be a two-step process in itself) and then relocated for final analysis in the MS step. The multi-step process increases analytic complexity and introduces potential for error to the protocol.
Moreover, the complexity and instrumentation sensitivity of mass spectrometry, as well as that of many of the possible ADI protocols, are not conducive to many analytical applications such as sample analysis in the field. Applications to date tend to be in the qualitative analysis of species adsorbed to surfaces, such as pesticides on foodstuffs, drug residues on currency, and small molecule components of bulk materials such as pharmaceuticals, with protocols requiring sample transport and preparation in the lab, which introduces the possibility of contamination and further complicates the analysis protocols.
What are needed in the art are analytical systems and methods that can provide accurate and consistent results with small sample sizes and that can operate at low cost and low energy consumption. A single-step analytic approach that can be portable for use in the field would also be of great benefit.